1. Field of the Invention
This invention relates to a magnetic recording medium. This invention particularly relates to a magnetic recording medium for use in recording a digital signal at high densities and reproducing the recorded digital signal. This invention more particularly relates to a coated type of magnetic recording medium having good storage stability.
2. Description of the Prior Art
Magnetic recording media have heretofore been used widely as sound recording tapes, video tapes, computer tapes, floppy disks, hard disks, and the like. Nowadays, there is a strong demand for the recording of magnetic information at high densities, and the recording wavelength is set to be short. Also, various recording systems, including the analog system and the digital system, have heretofore been studied.
In order to satisfy the demand for the recording of magnetic information at high densities, it has heretofore been studied to utilize thin metal film types of magnetic recording media, in which a thin metal film is employed as the magnetic layer. However, from the view point of productivity and reliability in practical use, such as corrosion resistance, it is advantageous to use coated types of magnetic recording media. The coated types of magnetic recording media comprise a substrate and a magnetic layer coated on the substrate, the magnetic layer comprising a binder and ferromagnetic particles dispersed in the binder.
The packing density of the magnetic material in the magnetic layer of the coated types of magnetic recording media is lower than the packing density in the magnetic layer of the thin metal film types of magnetic recording media. Therefore, the electromagnetic characteristics of the coated types of magnetic recording media were lower than those of the thin metal film types of magnetic recording media. However, recently, with the improvements made in ferromagnetic particles, coated types of magnetic recording media having excellent characteristics have been proposed.
In order to enhance the electromagnetic characteristics of the coated types of magnetic recording media, it is efficient, for example, to improve the magnetic characteristics of the ferromagnetic particles and to smooth the surface of the magnetic layer. In particular, as a technique for achieving the smoothing of the magnetic layer, a wet-on-wet coating technique has been used in practice. The wet-on-wet coating technique enables the enhancement of dispersibility of the magnetic particles and the formation of a very thin, smooth magnetic layer via a non-magnetic layer.
Also, with the rapid advances made in recording media in recent years, there has arisen a tendency toward long-term storage of enormous amounts of image signals and other kinds of signals. From the view point of capability of long-term signal storage, among the recording media, magnetic recording media are more advantageous than the other kinds of recording media. By the investigation of a change in magnetic characteristics and a change in input-output characteristics during long-term storage, it has been confirmed that magnetic recording media have good storage stability.
However, magnetic recording media for recording and reproducing video signals have advanced from 2-inch tapes to 1-inch tapes, to 0.5-inch tapes, and to 8 mm tapes, and the recording density has thus been set to be high. Also, the surface roughness of the magnetic layers of the magnetic recording media for recording and reproducing video signals has been set to be high. With respect to the surface roughness, it has been confirmed that the coated types of magnetic recording media have a certain level of reliability, including runnability, after long-term storage or storage under severe environmental conditions, and the coated types of magnetic recording media have the confidence of the users. However, as the coated types of magnetic recording media have been put into competition with vacuum deposited, thin-layered metal tapes (ME tapes), it has become necessary to solve the problems in that, in cases where the value representing the surface roughness of the magnetic layers of the coated types of magnetic recording media is at most 3 nm, the runnability of the coated types of magnetic recording media become low after long-term storage.
Heretofore, in order for the aforesaid problems to be eliminated, alkali and alkaline earth metals constituents have been removed as much as possible from the magnetic layer such that the formation of fatty acid Na salts and fatty acid Ca salts during the storage under environmental conditions of a temperature of 60.degree. C. and a relative humidity of 90% may be restricted. In particular, if the fatty acid Ca salts are formed in the magnetic layer, they will come out on the surface of the magnetic layer to form elevated protrusions. As a result, space loss between the magnetic recording medium and magnetic head will occur, which will result reduction of output signals. Therefore, the alkali and alkaline earth metals constituents are eliminated from the magnetic layer, and the formation of the fatty acid Ca salts is thereby restricted. However, the problems occur in that the other fatty acid metal salts, such as fatty acid Fe salts, are formed in the magnetic layer and come out to the surface of the magnetic layer, and therefore an increase in the kinetic friction coefficient of the surface of the magnetic layer still occurs after the storage under environmental conditions of a temperature of 60.degree. C. and a relative humidity of 90%. As for the problems with respect to the increase in the kinetic friction coefficient, no improvement has yet been made. It has been found that, in cases where the surface roughness of the magnetic layer is at most 3 nm, the increase in the kinetic friction coefficient of the surface of the magnetic layer after the storage occurs markedly.